The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. Embodiments of the present invention relates generally to wellbore cables.
In high-pressure wells, wireline is run through one or several lengths of piping packed with grease to seal the gas pressure in the well while allowing the wireline to travel in and out of the well. Insulated stranded conductors typically consist of several wires (typically copper) cabled at a lay angle around a central wire, with one or more layers of polymeric insulation extruded over the bundled strands. The insulation is not able to penetrate into the spaces between the conductor strands. Additional space is typically left between the central strand and the next layer of stranded wires, and between the insulation and the outer surface of the conductor wires, which create a potential pathway for high-pressure downhole gases. When the cable is being pulled out of the wellbore at high speed, these gases can decompress, leading to bulging insulation. If the gases decompress rapidly, this can even cause the insulation to burst, through the phenomenon of explosive decompression.
Problems with gas migration through interstitial spaces are also observed in coaxial cables and individual insulated conductors. In coaxial cables, a central, insulated conductor is covered in a served shield consisting of individual wires ranging in diameter from about 8 mm to about 14 mm. An additional jacket is placed over the served shield, followed by two layers of served armor wire. Because these wires do not “dig in” sufficiently to the central conductor's insulation, individual wires can become raised up above the other wires and “milk back” during the manufacturing process, damaging the cable. Individual wires can also cross over each other, causing high spots in the served shield, which can lead to similar damage. Because the served wires are not firmly affixed to the conductor, compression extrusion of the outer jacket layer would displace the shield wires. The tube extrusion methods that are compatible with unstable served shield wires leave gaps between the served shield and the outer jacket, which provide a pathway for pressurized downhole gas. The cable can be damaged when this pressurized gas is released through weak spots in the jacket through explosive decompression. It also compromises separation between the served shield and the armor wires.
Because the armor wire layers have unfilled annular gaps, gas from the well can migrate into and travel through these gaps upward toward lower pressure. This gas tends to be held in place as the wireline travels through the grease-packed piping. As the wireline goes over the upper sheave at the top of the piping, the armor wires tend to spread apart slightly and the pressurized gas is disadvantageously released.
In seismic cables used in offshore exploration, armors are typically placed around the cable's circumference at 50 to 60% coverage at a high lay angle (i.e., closer to perpendicular to the cable than other cables). Because of the space between the armors, the armors tend to milk or cross over one another during manufacture, and are not uniformly spaced. Non-uniform armor spacing can lead to weak spots in the completed cables. In gun cables, which carry extremely high air pressure, this is particularly disadvantageous.
One potential strategy to seal armor wires and prevent gas migration through the cable is known as “caging.” In caging designs, a polymer jacket is applied over the outer armor wire. A jacket applied directly over a standard outer layer of armor wire would essentially be a sleeve; this would be unacceptable under loading conditions. To create a better connection with the inner layers, space is created in the outer armor wire layer by reducing armor wire coverage from 98% to between 50 and 70%.
This type of design has several problems. When the jacket suffers a cut, potentially harmful well fluids enter and are trapped between the jacket and the armor wire, causing it to rust very quickly, which may cause failure if unnoticed and, even if noticed, is not easily repaired. Certain well fluids may soften the jacket material and cause it to swell. This swelling loosens the jacket's connection with the outer armor wire layer. The jacket is then prone to being stripped from the cable when the cable is pulled through packers, or seals, or if it catches on downhole obstructions. The jacket does not provide adequate protection against cut-through. Cut-through allows corrosive well fluids to accumulate in the annular gaps between the core and the first layer of armor wires. To improve bonding between the jacket and the outer armor wires, armor wire coverage must be significantly reduced. This means fewer or smaller outer armor wires are used. As a result, cable strength is also significantly reduced.
Because of the above problems, caged armor designs can only be used currently in piping/coiled tubing systems. Even in those applications, caged armor designs will experience several of the problems mentioned above. One current manufacturing strategy to maintain uniform armor spacing in seismic cables is to place filler rods (consisting of polymeric rods or yarns encased in a polymeric extrusion) between polymer-coated armor wires. While this helps to keep the armor wires in place and maintain spacing during the manufacturing process, it also creates more interstitial spaces between the armor wires and the spacer rods.